The present invention relates to a method preparing a composite oxide having especially excellent oxygen absorbing and desorbing capability which is utilizable for a catalyst for purifying exhaust gases and for functional ceramics.
Cerium oxide has hitherto been employed in large quantities as catalysts for purifying exhaust gas and as ceramics. For example, in the field of catalysts, cerium oxide is used for enhancing the purification rate exhaust gas components such as hydrocarbons, carbon monoxide and NOx, taking advantage of its characteristic properties of absorbing oxygen, in an oxidizing atmosphere and desorbing oxygen in a reducing atmosphere. In the field of ceramics, cerium oxide is used in the form of compounds or mixtures with other elements as electrically conductive ceramics, such as solid electrolytes, taking advantage of its characteristic properties mentioned above. Such cerium oxide heretofore known is usually prepared, for example by adding oxalic acid or ammonium bicarbonate to a solution of nitrate or chloride of cerium, filtering and washing the resultant precipitate, followed by drying and calcining.
However, the conventional composite oxide predominantly composed of cerium oxide prepared according to the above method, though it exhibits oxygen absorbing and desorbing capability, has a drawback in that it is incapable of sufficiently absorbing and desorbing oxygen at 400.degree. to 700.degree. C., and is deteriorated in performance after heating to a higher temperature.
Up to now, a cerium-zirconium composite oxide having high oxygen absorbing and desorbing capability is known and disclosed in Japanese Laid-open Patent Publication No. 5-105428 (1993), but development of a composite oxide exhibiting even more excellent oxygen absorbing and desorbing capability has still been desired.